1. Field of the Invention
The present invention generally relates to tilt correction for an object lens relative to an information recording medium in an optical disk recorder/player, and more particularly, to a method and apparatus for correcting a tilt relative to an information recording medium and to an optical disk recorder/player using such a tilt correcting technique.
2. Description of Related Art
With an optical disk apparatus (i.e., an optical disk recorder/player), a laser beam is guided onto the recording side of an information recording medium, such as an optical disk, to record information. The recorded information is reproduced based on the light reflected from the recording side. In general, the optical disk apparatus is provided with an optical pickup device, which emits a laser beam to form a light spot on the recording side of the information recording medium, as well as receiving light reflected from the recording side.
Generally, an optical pickup device has an optical system including an object lens, which guides a laser beam emitted from a light source onto the recording side of the optical recording medium and guides return light flux reflected from the recording side to a prescribed light-receiving position. The optical pickup device also has a light receiving element arranged at the light-receiving position to receive the return light flux. The light receiving element outputs not only information reproduced from the data recorded in the recording side of the medium, but also signals containing servo information required for positioning of the object lens and the optical pickup device itself.
In recent years and continuing, the recording density in information recording media is increasing in response to demand for increased recording capacities of information recording media. In order to increase the recording density, the diameter of the light spot formed on the recording plane has to be decreased. To achieve this, use of an object lens with a large numerical aperture is becoming popular. However, as the numerical aperture increases, influence of wavefront aberration due to offset of the optical axis of the object lens from the line perpendicular to the recording plane also increases. The offset of the optical axis of the object lens is referred to as “tilt”, which is likely to cause deformation of the light spot, as well as degradation of signals output from the light receiving element and containing reproduced information and servo information.
In general, information recording media are resin molded products, and most of the information recording media are fabricated by injection molding or similar techniques from the standpoint of productivity. In injection molding, molten resin is injected into a molding die having a cavity corresponding to the shape of the product to be molded, under pressure. If the product to be molded is a disk, such as an information recording medium, a radial flow type molding die having a direct gate (or an injection port) at the center of the cavity (corresponding to the rotational center of the information recording medium) is typically used. The molten resin injected through the direct gate flows from the center of the cavity toward the periphery. Because the temperature and the cooling rate of the resin injected into the cavity are non-uniform, internal stress is created and remains in the molded product. As a result, the resin density of the molded product becomes uneven. Consequently, the surface of the information recording medium is likely to become uneven. When such an information recording medium with an uneven recording plane is used, influence of wavefront aberration due to the tilt of the object lens varies depending on the location on the information recording medium.
To avoid the fluctuating frontwave aberration, various methods and apparatuses for correcting tilt have been proposed. For example, JPA 2001-52362 discloses a tilt servo apparatus, in which tilt correction is carried out for the data recorded area such that the amplitude of an RF signal becomes the maximum, while for the non-recorded area tilt correction is carried out such that the offset in the push pull parameter for detection of track error signals (hereinafter referred to as the “push-pull signal” for convenience) becomes substantially zero because RF signals cannot be acquired from the area in which no data are recorded.
Another publication JPA 2002-25090 discloses an optical disk apparatus, in which tilt correction is carried out such that the offset in the push-pull signal becomes substantially zero, or that the amplitude of the push-pull signal (which is referred to as the “traverse signal” in JPA 2002-25090) in traversing the track becomes the maximum.
In general, optical disk apparatuses are designed so that the tilt becomes substantially zero when the offset in the push-pull signal is zero or the amplitude of the push-pull signal in traversing the track is the maximum, in order to acquire the optimum reproduced signal. However, depending on the precision of the manufacturing equipment, the optical system of the optical pickup device is assembled at a position offset from the correct assembling position. In this case, even if the amount of the positional offset is within the acceptable range, the tilt may not be reduced to zero when the offset of the push-pull signal is zero or when the amplitude of the push-pull signal in traversing the track is the maximum. In other words, optimum tilt correction cannot be performed using the push-pull signal when the assembling precision varies.